The present invention generally relates to electrical switch circuits, and in particular to methods and circuits for increasing the turn-on time in switches having a MOS controlled thyristor (MCT) through the innovative use of a thyristor based device.
A thyristor is a four layer semiconductor switching device. The bistable action of a thyristor depends on PNPN regenerative feedback. A thyristor can be unidirectional or bidirectional. A thyristor can be triggered to conduction from a desired point within a quadrant of an applied AC voltage. A triac and silicon bilateral switch are common bidirectional thyristors. The silicon controlled rectifier (SCR) is the most common unidirectional thyristor; others include the gate-turn off switch (GTO) and light activated silicon controlled rectifier (LASCR).
A MOS controlled thyristor is a power semiconductor device that combines a MOS transistor as the gate and a thyristor as the power source. This composite device has the lowest forward voltage drop of any voltage-controlled power source, including power MOS field effect transistors (MOSFET) and insulated gate bipolar transistors (IGBT). MCT's are turned on quickly over their entire active area and have a very large di/dt capability. A description of such devices and explanation of their fabrication process may be found in U.S. Pat. No. 5,111,268 to V. A. K. Temple, which is hereby incorporated by reference. For example, a 600 volt, 75 amp MCT has a di/dt of up to 10,000 amps/microsecond and may be turned on in about 0.2 microseconds. MCT's are used in integrated circuit applications to handle high currents and high voltages. Such applications include high frequency switching regulated power supplies, motor drives and. solid state switches for power electronics building block (PEBB) modules of different configurations for general circuit application.
The most common switch configuration is the half bridge consisting of two switching devices in series, each switching device having an antiparallel diode. Although theoretically, it is desired that the power switch have infinite turn-on speed, the faster such power switch turns on, the faster the antiparallel diode of the series connected switch is turned off. The finite stored charge and the low innate capacitance of power diodes leads to large diode reverse recovery currents and voltage overshoots that often cause failure of the diode. This invention provides a means to effectively provide the appropriate slowing in the turn-on time of a power switch to save the antiparallel diode of the power switch in series therewith. Moreover, this slowing is accomplished by increasing the turn-on time of the power switch without requiring that such means be active in the main part of the power switch cycle.
In applications, particularly half bridges using MCT's as the two main series connected switches, the extremely fast and efficient turn-on of the MCT may stress the antiparallel diode of the series connected MCT beyond the normal capability of such diode and thus induce diode failure by the voltage snap during reverse recovery.
One solution to the diode failure problem is to increase module inductance to limit the rate of change of current di/dt in the power switch. However, this leads to much increased losses and overvoltage stress at turn-off.
Another possible solution, albeit one with some difficulty, can be achieved by making the inductance saturable so that it is high at turn-on and low at turn-off. However, such saturable reactors need careful design and add losses, weight, volume, and cost. They are also limited in temperature capability.
Another possible solution involves a semiconductor based solution which includes paralleling the series connected MCT's with either IGBT's or MOSFET's. The IGET or MOSFET would be turned on before the MCT and used to control the di/dt of the MCT. Unfortunately, this solution requires considerable additional semiconductor area and losses as IGBTs and MOSFETs are far less turn-on and conduction efficient than the MCT.
It is, accordingly, an object of the present invention to provide a novel method and circuit using a thyristor based device to protect a power electronics circuit from failure due to diode stress.
It is another object of the present invention to provide a novel method and circuit to reduce the stress on an antiparallel switch diode during switch turn-on.
It is yet another object of the present invention to provide a novel method and circuit to increase the turn-on time, not the turn-off time, and reducing the di/dt capability of a power electronics switch while retaining the circuit's performance when the switch is fully turned on.
It is still another object of the present invention to provide a novel method and circuit for protecting a power electronics circuits.
It is a further object of the present invention to provide a novel method and circuit for controlling MOS controlled thyristor switches.
It is yet a further object of the present invention to provide a novel MCT power electronics switch having a slower turn-on time and current rate.
It is still a further object of the present invention to provide a novel semiconductor switch having an MCT connected in parallel with a smaller MCT and an inductor, and integrated in a single semiconductor device.